As a technique for satisfying both low rolling resistance and high braking performance on wet road surfaces, it is effective to use silica as a filler. Here, it is indispensable to use a silane coupling agent together with silica for securing further loss reduction and abrasion resistance. In addition, a silane coupling agent plays a role to prevent a vulcanization accelerator from adsorbing to the surface of silica through interaction thereof with the surface of silica.
However, in the case where a silane coupling agent is used, an unreacted component may remain in rubber, thereby often causing rubber scorching during kneading. Consequently, it is known to use a small amount of a silane coupling agent in combination with a nonionic surfactant to give a rubber composition excellent in loss reduction, wet performance, abrasion resistance, workability and the like, without causing rubber scorching (see PTL 1).
However, in this case, since the nonionic surfactant adsorbs rapidly to the surface of silica, the reaction between the silane coupling agent and silica is inhibited. As a result, the capability of silica to reinforce the rubber component is lowered, and the abrasion resistance of the rubber component is thereby worsened.
PTL 2 proposes a rubber composition containing, as basic components, at least (i) one diene elastomer, (ii) a white filler as a reinforcing filler and (iii) an alkoxysilane polysulfide as a coupling agent (white filler/diene elastomer) along with (iv) an enamine and (v) a guanidine derivative.
PTL 3 discloses a rubber composition containing, as basic components, at least (i) one diene elastomer, (ii) a white filler as a reinforcing filler and (iii) an alkoxysilane polysulfide as a coupling agent (white filler/diene elastomer) along with (iv) zinc thiophosphate and (v) a guanidine derivative.
PTL 4 describes a rubber composition containing, as basic components, at least (i) a diene elastomer, (ii) an inorganic filler as a reinforcing filler and (iii) an alkoxysilane polysulfide (PSAS) as an (inorganic filler/diene elastomer) coupling agent, as combined with (iv) an aldimine (R—CH═N—R) and (v) a guanidine derivative.
Further, PTL 5 proposes a rubber composition basically containing at least (i) a diene elastomer, (ii) an inorganic filer as a reinforcing filer and (iii) an alkoxysilane polysulfide as a coupling agent, as combined with (iv) 1,2-dihydropyridine and (v) a guanidine derivative.
As a case of increasing the activity of the coupling function of a silane coupling agent in consideration of kneading conditions, there is mentioned PTL 6.
In these inventions, however, the reaction between the silane coupling agent and the rubber component is not sufficient, and therefore there is still room for improving them in point of enhancing low-heat-generation property.
Recently, in association with the movement of global regulation of carbon dioxide emission associated with the increase in attraction to environmental concerns, the demand for low fuel consumption by automobiles is increasing. To satisfy the requirement, it is desired to reduce rolling resistance relating to tire performance. As a means for reducing the rolling resistance of tires, there is mentioned a method of applying a low-heat-generating rubber composition to tires.
As a means for obtaining a low-heat-generating rubber composition, in the case of a synthetic diene rubber, there is mentioned a method of using, as the rubber composition, a polymer having an increased affinity to carbon black and silica (for example, see PTL 7). On the other hand, in the case of a natural rubber, there is mentioned a method of incorporating a highly-reactive carbon black into a modified natural rubber prepared by modifying a natural rubber (for example, see PTL 8).
According to PTLs 7 and 8, by increasing the affinity of the rubber component to the filler such as carbon black or the like, the heat-generation-property of the rubber composition can be lowered and accordingly there can be obtained tires having a low hysteresis loss.
However, with further advanced reduction in fuel consumption by automobiles, further improvement of low-heat-generating tires has become desired.